No, an average reader of this FB post cannot "make their own conclusions". They are not physicians; they are not dermatologists; they are not toxicologists. The responsible thing to do (if they want to turn Navalny into a public medical case study) would be to lay out differential diagnosis and to explain what alternatives to poisoning have been considered and why poisoning cannot be ruled out (if that’s what they are saying).
I remember that there is (was?) a difference in dermatological terminology between the Russian medical tradition and the English-speaking world; but in the latter, contact dermatitis includes irritant contact dermatitis and allergic contact dermatitis. Writing contact dermatitis, not otherwise specified (unless the meaning of the term is different in the Russian tradition) does not differentiate between a reaction to a toxin and a reaction to an allergen. Saying "his cellmates are fine, and only he got sick" completely ignores how allergies work. If an allergy has been ruled out, the post should explain based on what evidence.
And maybe, just maybe, after such a detailed explanation the readers will be able to "make their own conclusions".